Large aperture, space-based telescopes can benefit from the use of a transmissive diffractive optic as the primary collecting element. For example, such elements are advantageous in terms of mass efficiency and stowed size for launch vehicle integration. However, the dispersion of light wavelengths (i.e. chromatic aberrations) by the diffractive primary optical element must be corrected by a secondary diffractive optic if the telescope is to be used in a broad-band imaging application.
The correction of chromatic effects can be performed in various ways. For example, systems that correct chromatic effects from a Fresnel lens by collecting data at multiple detectors arrayed along the optical axis, and then digitally processing the data to obtain composite, multi-color images, or spectral-selected images, have been described. However, because relatively little light is typically available, such an approach would suffer from negative effects caused by noise in the detectors. In addition, significant processing power would be required.
The use of a corrector diffractive optical element has also been discussed. However, and particularly in a space-based telescope, it is desirable to minimize the size and mass of the corrective optic. Achieving correction of chromatic dispersion using a relatively small and low mass assembly has been difficult. In particular, such systems have required relatively large structures because they would include a very small angle of incidence at a connector element and a large exit angle.